1. Field of Invention
The invention relates generally to optical devices and more particularly to Wavelength Division Multiplexing (WDM) systems for compensating a group delay experienced from multiple wavelengths.
2. Description of Related Art
Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiple wavelengths may be transmitted along the same optical fiber. This totality of multiple combined wavelengths comprises a single composite transmitted signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component wavelengths, or “channels”, typically by a dense wavelength division multiplexer. This separation must occur in order for the exchange of wavelengths between signals on “loops” within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths.
Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
Conventional methods used by dense wavelength division multiplexers in separating an optical signal into its component channels include the use of filters and fiber gratings as separators. A “separator,” as the term is used in this specification, is an integrated collection of optical components functioning as a unit which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the optical art.
When transmitting multiple wavelengths, it will typically cause what is known as “a group delay”, which is the delay difference between sending a first wavelength through a first path to a destination and sending a second wavelength through a second path to the destination, where the first wavelength produces a first travel time while the second wavelength produces a second travel time. To compensate for the group delay between the first and second wavelengths, a prior equalizer design employs two WDM 3-port devices in which each WDM device has two ports on one side and a single port on the other side, as shown in FIG. 1. A plurality of wavelengths or N lambda (λ1, λ2 . . . λn) 110 is transmitted into a first port 121 in a first WDM device 120 having a filter 125. The wavelength λ1 111 passes through the filter 125 and to a third port 123 in the first WDM device 120 and onto a first fiber path L1 130 and continues along the first fiber path L1 130 to a first port 161 in a second WDM device 160. The remaining wavelengths λ2 . . . λn 112 are reflected back through a second port 122 in the first WDM device 120 toward a second fiber path L2 140 and continues to a second port 162 in the second WDM device 160. The first wavelength λ1 111 and the reflected wavelengths λ2 . . . λn 112 are combined at the second WDM device 160 to produce a transmitted optical signal comprising λ1, λ2 . . . λn 115 through a third port 163.
The total group delay is calculated by subtracting the delay through the first fiber path L1 130 from the second fiber path L2 140, and the δ constant which reflects the adjustment length inside of the two devices, represented mathematically as the group delay (λ1, λ2 . . . λn) G=L2−L1−δ. The first fiber path L1 is computed by multiplying the travel speed c by the travel time t(c*t), where the symbol c denotes the travel speed and the symbol t denotes the travel time. If the length of fiber in the first fiber path L1 130 or the second fiber path L2 140 is longer, it will translate into a longer amount of travel time.
Competitive pricing among of optical device and system companies encourages new and innovative designs that perform the same or greater functions while reducing the cost of the optical component or system. Accordingly, it is desirable to have a group delay equalizer that performs the same or similar functions but at a lower design and manufacture cost.